1. Field of the Invention
The invention relates to apparatus for molding thermoplastic tubing in which a sizing plug is used to define internal diameter of the tubing and, more particularly, to apparatus for profile thermoplastic tube formation, including a sizing plug possessing suction means for differential suction distribution over the sizing plug surface to minimize rhythmic tubing deformation during production. The invention is especially useful in the production of profile tubing such as ribbed or double walled tubing.
2. Background
The apparatus may be of the type in which tubing of thermoplastic material is continuously extruded into a travelling tubular mold tunnel about a mandrel and is confined to the shape of the pipe by a sizing plug downstream of the mandrel. Such a plug may, on some occasions, when extrudate does not easily flow fully into the mold, be heated to increase the fluidity of the extrudate so that it may more easily flow into recesses of the mold, for example, under the influence of suction from the bases of the recesses. However, more usually the extrudate is provided in sufficiently molten condition to flow fully into the mold. In this case, it is necessary to provide a sizing plug to define the inner wall of the pipe so as to confine the extrudate in an appropriate casting cavity.
A simple sizing plug defining the inner wall of a tube is describe by Chaplain in U.S. Pat. No. 4,365,948 issued Dec. 28, 1982. However, some temperature control is usually desirable in a sizing plug and Lupke in U.S. Pat. No. 4,545,751 issued Oct. 8, 1985, describes such an arrangement. In that arrangement the sizing plug does not define an inner wall of the pipe but distributes air into the interior of the corrugated tubing mold. Pressured air is delivered through a central core coaxial with an extrusion nozzle. Hot peripheral air is drawn off and central cool air is permitted to exit the plug. Such arrangement is somewhat limited due to the limited range of temperature in cooling. The arrangement is less suitable where ribbed tubing is to be produced since it is desirable for the plug to contact the inside wall of the pipe to confine it into a casting region. Due to the need for direct contact, air cannot be released between the plug wall and the inner pipe wall for cooling.
A cooling plug suitable for use in the formulation of ribbed pipe is disclosed by Lupke in his later U.S. Pat. No. 4,555,230 issued Nov. 26, 1985. In that patent, Lupke does not release cooling air but provides a helical coil of tubing through which cooling fluid may be passed just under the follower plug surface.
Production of non-profile or plain single walled tube usually involves extruding a parison into a mold, the temperature of which may be adjusted by the use of a water jacket. Generally, no sizing plug is used to form the inner surface of the tube. Rather, gas pressure or the like is used to press the tube against the outer mold. However, the inner wall of the tube thus formed may not be totally even due to a variety of reasons, such as uneven shrinkage during setting of the tube.
Canadian Patent Application No. 586,828 to the inventor herein, filed Dec. 20, 1988, describes a cooling plug in which cooling of the plug is by isentropic expansion of gas into the plug itself. This has various advantages in facilitating transport of cooling fluids to the plug and controllability of the degree of cooling. However, the problem of rhythmic deformation and indentations in the inner tube wall are not specifically addressed.
Even though sizing plugs have been provided to form a smooth inner wall in the tube, when ribbed pipe is formed, some problems have been experienced in obtaining a smooth inner wall. The reason for this is the variation in the requirement of plastic material due to the varying wall thicknesses for the ribs and troughs. More plastic mass is required to form the ribs than troughs, that part of the tube wall between the ribs. In addition to the mechanical stresses in the apparatus at the point of filling the mold blocks due to this varying demand for plastic material, uneven shrinkage results from the unequal cooling of the larger plastic mass (ribs) and smaller mass (troughs).
U.S. patent application No. 405,777 of the same inventor, filed Sep. 11, 1989, describes and claims the use of an accumulator chamber for plastic material at the point of mold filling. The accumulator chamber is intended to even out pressure variations and reduce stresses. The use of such an accumulator chamber is very useful but there still may be unevenness present on the inside wall of the tube due particularly to the latent cooling effects.
Thus, the inside wall of the tube may have a slight wave formation, tending to bulge inwardly between the ribs where the pressure of extrudate is greater. Due to contraction upon cooling, the inside tube surface tends to actually recede, thus forming an indentation opposite a rib where the pressure of extrudate is less. While the undulating effect is minimized with the use of an accumulator chamber, as described above, the tube fabrication conditions and thermoplastic heat-capacity characteristics may prevent elimination of rhythmic bulging.
In other profile tubing, such as double walled corrugated tubing, similar difficulties have been encountered due to pressure differences within the corrugations. If the pressure is too low, the inner tube wall may bow into the corrugation to thus form an undesirable concavity in the inner wall. If the pressure is too high, on the other hand, the inner wall may bow away from the corrugation forming an undesirable bulge in the inner wall.
The art contains descriptions of various approaches to generate a smooth inner tube wall. None, however, describe or suggest apparatus embodying the concept of applying diminishing suction in the upstream direction on the sizing/cooling plug surface, especially as applied to profile tubing extrusion. Such approaches are contained in, for example, Takada et al., U.S. Pat. No. 4,663,107, which describes a cooling fluid plug surface distribution system. Clearly, Takada et al. is not interested in profile tubing since it illustrates in FIGS. 2 and 3 the contraction of tube 7 downstream of mandrel 12. Branscum, U.S. Pat. No. 3,182,108, describes applying suction at the upstream end of a mandrel surface to remove steam vapors and the like. Chan et al., in U.S. Pat. No. 4,808,098, describes applying suction in describe apparatus for applying vacuum of diminishing strength upstream.